The present invention relates to an improvement in punch retainers for use in a punch and die assembly.
Punch retainers are used in the prior art to retain a punch in a die shoe when the punch is moved to form an aperture in a sheet of metal. Typically, ball-lock punch retainers spring-bias a ball into a notch in the punch. In many prior art retainers a backing plate is attached, using screws or other non-permanent methods, to a retainer body to dissipate reactive forces from the punch down it is forced into a piece of metal, to form an aperture in the metal. These prior art retainers include a number of passages through which dowels extend to properly align the punch retainer with an upper die shoe of a punch and die assembly. Problems are encountered with this type of prior art retainer since the retainer body is connected to the backing plate after formation of the individual parts. The various passages that extend through the retainer body and the backing plate are often improperly aligned and require close attention by an operator to assemble the punch retainer to a die shoe, which is inefficient. There is often waste since a particular backing plate may not be utilized with a particular punch retainer if the passages in the two can not be properly aligned.
Some prior art punch retainers solved this problem by eliminating the backing plate. An example of such a prior art retainer is illustrated in U.S. Pat. No. 3,563,124. In this patent, a plug is utilized in place of a backing plate to dissipate the force received from the punch. The force-dissipating plug must be aligned with the rear of the punch so that the reactive force transmitted into the punch will be transmitted into the plug. This patent addressed the alignment problem inherent in the previously discussed prior art by having a dowel aligned with the punch extend through the plug and into the die shoe. By eliminating the backing plate, the problem of achieving a number of properly aligned passages through both a retainer body and a backing plate is eliminated. A similar device is shown in U.S. Pat. No. 3,589,226.
Problems are still encountered with this type of punch retainer. The forces that must be dissipated from the punch are often of relatively large magnitude, and the plugs disclosed in the above-mentioned patents extend for a relatively small surface area. These plugs sometimes may not adequately dissipate a force, since they do not extend over an adequate surface area.
In addition, prior art retainers are also impractical since it is difficult to secure the spring which biases the ball into a spring passage. Special springs are often required which are more expensive than standard springs.
It is an object of the present invention to disclose a punch retainer which utilizes a backing plate such that an adequate surface area is achieved for force-dissipating means; at the same time not requiring alignment of passages within a retainer body and a backing plate when attaching the punch retainer to a die shoe. In addition, the present invention discloses a punch retainer that does not require special springs to bias the ball into the punch.